Because it might not work out so well for them? Let's not kid ourselves, this would be an invasion by sea against an opponent who's prepared for many years, has a modern defense force, and has explicitly designed a defense protocol to inflict as much damage as possible. This is not Russia stepping over fields they already encircle that are governed by a dysfunctional government. And it's not Isreal being allowed to bomb impoverished children with impunity either. And neither of those scenarios have worked out splendidly for them either.
Slightly less compressed, it should be "When a nearby state keeps saying they own you, then take the credible threat seriously and pretty much pretend they're going to act on that, even if you think it would be stupid of them to do that, because history is full of examples of states doing this kind of thing. Prepare accordingly".